Glycerol-induced Hyperhydration (GIH=21.4 ml/kg of glycerol solution, 47g/liter) will be tested as a countermeasure for the orthostatic tolerance lost as a result of HOI. The HOI procedure will serve as a model for the cephalad shift in body fluids which accompanies space travel. The experiments will involve (1) ingesting ad libitum water, (2) GIH prior to HOI, (3) ingesting glycerol Sol' after HOI to replace fluid lost during HOI, and (4) ingesting water after HOI to replace fluid lost during HO.